The use of microwave energy radiated within an enclosure to process materials has been in wide use for many years. The molecular agitation within the material resulting from its exposure to microwave energy provides frictional heat to cook or dry the material in a time period that is relatively short compared with other means of conventional heating. Typically, magnetrons and high power vacuum triodes provide the source of microwave energy.
Government agencies have allocated frequency bands centered at 915 and 2450 megahertz for use in microwave heating systems. The intensity of microwave energy permitted to leak from domestic and/or industrial microwave heating systems is restricted to less then 10 milliwatts per square centimeter. In the United States, for example, the Department of Health, Education and Welfare requires that microwave energy leakage from a domestic oven must not exceed 1 milliwatt per square centimeter in the factory or 5 milliwatts per square centimeter in the home. Further, the Occupational Safety and Health Administration requires a microwave energy exposure of less than 10 milliwatts per square centimeter. The International Microwave Power Institute has adopted a standard for intensity of microwave energy radiation leakage which is "less than 10 milliwatts per square centimeter". Accordingly, systems employing the use of microwave energy for processing of materials must have a means to prevent the leakage of the microwave energy from the enclosure.
Many industrial microwave heating applications require that there be access apertures into the enclosure so that materials may be transported therethrough with a conveyor. The suppression of microwave energy from these apertures has presented a problem which is more complex than the suppression of microwave energy from a batch oven.
One prior art approach to the suppression of microwave energy from a conveyorized microwave system was to line the inner walls of a tunnel which surrounded the aperture with a lossy material. The product would pass through the tunnel on a conveyorized system and the microwave energy would be absorbed by the lossy material. However, for large apertures, the system efficiency is relatively low because a substantial amount of energy is consumed by the lossy material. Further, if the mutually orthogonal dimensions of a cross-section of the tunnel are relatively large compared to a free space wavelength of the energy, the tunnel has to be prohibitively long to be an effective seal.
Another prior art approach to the problem is the use of a plurality of thin metal flaps that hang in the tunnel which has lossy walls. The flaps are pushed aside by the product as it passes through. The flaps however do not provide an effective seal when the tunnel cross-sectional mutually orthogonal dimensions are substantially greater than a free space wavelength of the microwave energy or when product pushing aside the flaps is not sufficiently lossy.
Another prior art approach is the use of reflectors which have dimensions substantially the same as the inner dimensions of the tunnel, said reflectors being attached to the conveyor system so that they substantially block the area of the tunnel as they pass through. Further, in U.S. Application Ser. No. 872,189, assigned to the same assignee herein, there is disclosed the use of a means for substantially preventing the transmission of the energy in the gap between the inner walls of the tunnel and the reflectors in a direction which is substantially perpendicular to the length of the tunnel. This approach, however, has two major disadvantages. First, because the reflectors are attached to the conveyor system, the reflectors themselves make it difficult to end load the conveyor. Accordingly, product loaded onto the conveyor by automated means must enter from the side, making the loading means of complex design to avoid contact with the moving reflectors. Second, because the reflectors are generally of a reflective material and structurally solid, the conveyor must be of a very heavy and rigid design to support the reflectors over the long continuous path.